Pancreatic beta-cell growth and function adapt to changing physiologic demands of the host, but the mechanisms regulating these facultative responses remain unclear. Inadequate adaptation leads to beta-cell failure, and can promote pathogenesis of diabetes mellitus. Physiologic signals like insulin and glucose are potent stimulators of beta-cell proliferation and the signaling pathways downstream of these ligands are well- studied. However, the mechanisms for transducing these cues into genetic changes that culminate in adaptive beta-cell proliferation remain undiscovered. The nuclear factor of activated T-cells (NFAT) proteins are transcription factors whose activation is regulated by intercellular signals, and by calcineurin, a calcium- dependent phosphatase. Recent studies suggest that NFAT proteins in beta-cells are crucial for regulating beta- cell gene expression, proliferation, and function. Demonstration of NFAT activation by beta-cell mitogens and identification of relevant NFAT genetic targets should reveal mechanisms underlying growth regulation of adult pancreatic beta-cells. The goal of this proposal is to elucidate the molecular and in vivo functions of calcineurin-dependent NFAT signaling that control islet development and facultative growth. Calcineurin inactivation in mice results in beta-cell failure, with impaired beta-cell proliferation, decreased insulin production, and reduced beta-cell mass. Mice lacking beta-cell calcineurin function develop overt diabetes. Strikingly, expression of activated NFATc1 in beta-cells lacking calcineurin restores beta-cell growth and prevents diabetes. Experiments in this application will test the hypothesis that NFAT governs expression of genetic targets that control beta-cell cycle, proliferation, and function. This proposal's specific aims are to: (1) Determine if glucose or insulin regulate NFAT function in beta-cells, and test if experimental NFAT activation is sufficient to prevent beta-cell failure (2) Identify target beta-cell genes whose expression is regulated by NFAT, and to reveal mechanisms of transcriptional control (3) Use novel conditional-genetic methods to control calcineurin and NFAT signaling in the embryonic mouse pancreas. These in vivo studies will reveal how NFAT controls gene expression, growth, and differentiation of beta-cells in development. Our genetic, molecular, and cellular analyses of calcineurin and NFAT function will add to the understanding of the mechanisms governing growth and differentiation of beta-cells. Thus, these studies may lead to new diagnostic, prognostic, or therapeutic strategies for a broad range of human diseases, including neuro-endocrine tumors, and diabetes mellitus.